The present disclosure generally relates to systems and methods for reducing nitrogen oxides (NOX) emissions, and more particularly, to systems and methods that split an exhaust stream to produce various feedstreams that are then recombined and fed to a selective catalytic reaction bed.
An internal combustion engine, for example, transforms fuel such as gasoline, diesel, and the like, into work or motive power through combustion reactions. These reactions produce byproducts such as carbon monoxide (CO), unburned hydrocarbons (UHC), and nitrogen oxides (NOX) (e.g., nitric oxide (NO) and nitrogen dioxide (NO2)). Air pollution concerns worldwide have led to stricter emissions standards for engine systems. As such, research is continually being conducted into systems and methods for reducing at least the nitrogen oxides emissions.
One method of removing nitrogen oxides from an exhaust fluid involves a selective catalytic reduction (SCR) process in which nitrogen oxides are reduced. For example, an ammonia-SCR process is widely used, wherein ammonia is used as a reducing agent in the selective catalytic reduction process to produce nitrogen gas and water. Ammonia-SCR, also referred to as NH3-SCR, is commonly used because of its catalytic reactivity and selectivity. However, practical use of ammonia has been largely limited to power plants and other stationary applications. More specifically, the toxicity and handling problems (e.g., storage tanks) associated with ammonia has made use of the technology in automobiles and other mobile engines impractical. For example, current regulations with regard to ammonia slip in vehicle exhaust systems are oftentimes difficult to meet.
The selective catalytic reduction of nitrogen oxides with hydrocarbons (HC-SCR) has also been exhaustively studied in recent years as a potential competitor to the NH3-SCR process. The hydrocarbon reductant reacts with the nitrogen oxides in the exhaust stream to form primarily nitrogen gas and carbon dioxide. The main advantage of this selective catalytic reduction process is the use of hydrocarbons as the reducing species as opposed to ammonia, which has minimal concerns with regard to slippage. For example, an HC-SCR process using propane as the reducing agent is known. However, the process has not been commercialized yet because of its low removal activity of nitrogen oxides. In addition, the catalysts used in the HC-SCR process generally present a narrow operating temperature range. As such, these types of beds are impractical in processing exhaust fluid streams generated from fuels in transient applications where the catalyst material is subjected to a broad range of temperatures.
Accordingly, a continual need exists for improved systems and methods for reducing nitrogen oxide emissions.